Oblique incident illumination of objects to be examined is used, for example, in wafer inspection to image surface structures of a specimen with high contrast and three-dimensional appearance using the diffraction effects produced at said surface structures. In German Patent DE 35 27 426 C1, an incident light illuminator for a microscope is described which can be used for this purpose and is adapted to provide oblique incident illumination and which uses an aperture stop of variable size. This patent presents an aperture stop device whose aperture stop can be displaced sideways from the optical axis in both directions. An increase in the distance from the optical axis is associated with an increase in the so-called “incident light angle” at which the axis of the illumination beam path meets the object plane (with respect to the normal to the object plane) after passage through the microscope objective.
However, the design of the incident light illuminator proposed in the above-mentioned document turns out to be mechanically complex and requires training and experience of the user to be successfully used in the field. In addition, as described in the document, the pivoting range may be limited depending on the position within the stand. Finally, as also described in the document, the reproducibility of the direction of light incidence (according to the pivoting movement of the aperture stop device) is limited. The rotation of the adjusting nut required for varying the incident light angle makes it virtually impossible for said angle to be continuously varied during the microscopic examination.
A technically simple and therefore economic alternative to interference microscopy, which is used to advantage for phase objects, is provided by reflected light microscopy with oblique illumination, which is suitable in particular for surface relief observation. To this end, generally, incident light Köhler illumination is used. In this connection, unilateral oblique illumination is obtained by positioning the aperture stop off-center (see document DE 35 27 426 C1 discussed above), while oblique illumination from all sides is obtained by inserting central or annular stops in the plane of the aperture stop.